To support the penetration of renewable energy generation in the power grid and to provide ancillary functions for system operation, the demand for energy storage systems (ESSes) has greatly increased. Batteries have a relatively large energy density and are commonly used in this application. However, batteries have a small power density and cannot be rapidly charged or discharged without harming the life time. This limits the performance of the battery based ESS. In contrast, UltraCapacitors (UCs) have a large power density, and can provide instantaneous high charge and discharge power. However, UCs have a low energy density and cannot provide energy in a long time frame. A Hybrid ESS (HESS) that has both battery and UC combines the advantages of the two energy sources, thus provides excellent power and energy capabilities. Meanwhile, such a hybrid system can effectively extend the battery life by reducing the charging and discharging cycles.
There are generally three types of circuit topologies for HESS using batteries and UCs. In a first topology, the battery and UC are directly connected at a direct current (DC) side of an inverter. Because the voltages of the battery and the UC are always the same in this topology, the power flow of the two sources cannot be controlled independently.
As an improvement, a DC/DC converter is inserted between the battery and the UC, so that the power of one of the energy sources can be controlled directly. By connecting a DC/DC converter to each energy source, the power of both energy sources is directly controlled. However, the additional DC/DC converters increase the cost of the system and, at the same time, introduce additional power loss. Furthermore, with increased power and energy ratings of the HESS for gird-level applications, the power semiconductor devices and passive components involved may become unsuitable to handle high voltages and currents.
For high power ESS, the topology of a Modular Multilevel Converter (MMC) has become a promising candidate. The modular structure of the MMC enables the usage of low voltage and high performance switching devices, provides an easy way to add redundancy to the system, and is scalable to different voltage and power levels. In addition, the multilevel output waveform decreases the total harmonic distortion, shrinks the size of the output filter, and increases the system efficiency by reducing the switching frequency. For ESS applications, in some solutions, a battery is integrated with a half bridge in each sub-module, and there is no power source connected to the dc bus. In this configuration, the battery life time will be affected by the large low frequency current flowing through the sub-module. To solve this issue, a DC/DC converter is inserted between the battery and the half bridge in each sub-module. However, this configuration significantly increases the number of switches and passive components in the circuit. For HESS applications, a MMC has been proposed with both battery and UC. The battery is put at the DC link, while the UC is integrated in the sub-module with a DC/DC converter and a half bridge. However, as described above, this configuration uses a large number of switches and passive components.